The growth of marine organisms on the submarine parts of a ship's hull increases the frictional resistance of the hull to passage through water, leading to increased fuel consumption and a reduction in the speed of the ship. Marine growths such as barnacles and algae accumulate so rapidly that the remedy of cleaning and repainting as required is impractical. In general, fouling by marine organisms is limited by applying to the hull a top coat of a paint which releases a biocide for the marine organisms. Traditionally such paints have comprised a relatively inert binder with a biocidal pigment which is leached from the paint. The main binders which have been used are vinyl resins, particularly a vinyl chloride/vinyl acetate copolymer, and rosin. The vinyl resins are sea-water-insoluble and paints based on them use a high pigment concentration so that there is contact between pigment particles to ensure leaching. Rosin is a hard brittle resin which is very slightly soluble in sea water. The biocidal pigment is gradually leached out of the matrix of rosin binder in use, leaving a skeletal matrix of rosin which becomes washed off the hull surface in thin flakes to allow leaching of the biocidal pigment from deeper within the paint film. Examples of rosin anti-fouling paints are described at pages 261 and 262 of the book "Recommended Practice for the Protection and Painting of Ships" published by the British Ship Research Association in 1973. Leaching of the biocidal pigment leaves an irregular layer of binder from which the pigment has been leached so that the roughness of the hull surface gradually increases in use.
The most successful paints in recent years have been the "Self-polishing copolymer" paints based on a polymeric binder to which biocidal triorganotin moieties are chemically bound and from which the biocide moieties are gradually hydrolysed by sea water, as described for example in British Pat. No. 1,457,590. This generates a water-soluble resin which is gradually dissolved from the hull surface by the movement of the ship in service. This gradual dissolution allows the painted hull to at least retain its initial smoothness and even to become smoother in service. Moreover, the gradual dissolution means that a fresh anti-fouling paint surface is continuously revealed, prolonging the time for which biocide release at the paint surface is maintained above the lethal concentration. However, there has recently been concern about the environmental effect of triorganotin biocides released from yachts in particular and there is a demand for a marine paint which dissolves smoothly in water without necessarily releasing triorganotin moieties.